1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device with improved aperture ratio and reduced parasitic capacitance.
2. Discussion of the Related Art
LCD devices, one of a number of flat display technologies, have improved performance characteristics including high contrast ratio, great gray level, high picture quality, and low power consumption.
For example, the LCD device having a thin profile may be fabricated in a wall-mountable type for a monitor of a television. Also, because the LCD device achieves light weight and low power consumption, the LCD device may be applicable as monitors for notebook computers, a personal computers, television, and aircraft.
The LCD device generally includes a thin film transistor array substrate, a color filter array substrate, and a liquid crystal layer. In particular, the thin film transistor array substrate includes a thin film transistor, a pixel electrode, and a storage capacitor in a pixel region defined by a gate line and a data line. Also, the color filter array substrate includes a color filter layer and a common electrode. The liquid crystal layer is formed between the thin film transistor array substrate and the color filter array substrate. By applying a voltage to the electrodes, liquid crystal molecules of the liquid crystal layer are aligned, thereby controlling the amount of light transmitted to displaying images.
A related art LCD device will be described with reference to the accompanying drawings.
FIG. 1 is a plan view of a related art LCD device. FIG. 2 is a cross sectional view of a related art LCD device along I-I′ of FIG. 1.
A related art LCD device includes a thin film transistor array substrate 111, a color filter array substrate 121, and a liquid crystal layer 131. The thin film transistor array substrate 111 and the color filter array substrate 121 are bonded to each other at a predetermined interval, and the liquid crystal layer 131 is formed between the two bonded substrates 111 and 121. Also, because the LCD device does not emit light, it requires an additional light source, for example, a backlight 150. The backlight 150 is provided under the thin film transistor array substrate 111.
As illustrated in FIG. 1 and FIG. 2, the thin film transistor array substrate 111 includes a gate line 112, a data line 115, a pixel electrode 117, a thin film transistor TFT, and a storage capacitor. The gate line 112 and the data line 115 are formed perpendicular to each other to define a pixel region. Also, the pixel electrode 117 is formed in each pixel region to apply a signal voltage to the liquid crystal layer 131 according to a data signal. Then, the thin film transistor TFT is formed at a crossing portion of the gate line 112 and the data line 115, wherein the thin film transistor TFT is turned-on/off based on a scanning signal applied to the gate line 112 to transmit the data signal applied to the data line to the pixel electrode 117. The storage capacitor is formed to decrease a level-shift voltage, and to sustain pixel information during a non-select period.
In addition, a gate insulating layer 113 is formed between the gate line 112 and the data line 115, and a passivation layer 116 is formed between the thin film transistor TFT and the pixel electrode 117.
Accordingly, the thin film transistor TFT is comprised of a gate electrode 112a, the gate insulating layer 113, a semiconductor layer 114, a source electrode 115a, and a drain electrode 115b. The gate electrode 112a is diverged from the gate line 112, and the gate insulating layer 113 is formed on an entire surface of the thin film transistor array substrate 111 including the gate electrode 112a. Then, the semiconductor layer 114 is formed on the gate insulating layer above the gate electrode 112a. Also, the source electrode 115a is diverged from the data line 115 and is overlapped with one side of the semiconductor layer 114. The drain electrode 115b is overlapped with the other side of the semiconductor layer 114, and the drain electrode 115b penetrating the passivation layer 116 is electrically connected with the pixel electrode 117, thereby applying a voltage to the pixel electrode.
A storage capacitor Cst is comprised of a capacitor electrode 126, the pixel electrode 117, the gate insulating layer 113, and the passivation layer 116. The capacitor electrode 126 is formed on the same layer as the gate line 112 in parallel. Also, the gate insulating layer 113 and the passivation layer 116 are interposed between the capacitor electrode 126 and the pixel electrode 117. Thus, the storage capacitor sustains electric charges of the liquid crystal layer during a turning-off block of the thin film transistor TFT.
Although not illustrated, the storage capacitor may be formed above the gate line by using a predetermined portion of the gate line as the capacitor electrode.
The storage capacitor has a structure of forming an insulating layer between capacitor lower and upper electrodes. In this case, the capacitor electrode 126 serves as the capacitor lower electrode, the gate insulating layer 113 and the passivation layer 116 serve as the insulating layer, and a predetermined portion of the pixel electrode 117 overlapped with the capacitor electrode 126 serves as the capacitor upper electrode.
The gate insulating layer 113 is formed of an inorganic insulating material such as silicon nitride SiNx or silicon oxide SiOx, having a dielectric constant of about 7.5, at a thickness between 1500 Å and 5000 Å. Also, the passivation layer 116 is formed of an organic insulating material such as BCB (BenzoCycloButene) or acrylic resin, having a low dielectric constant of about 3.4, at a thickness between 3 μm and 5 μm.
When the passivation layer 116 is formed of the organic insulating material such as BCB, it is possible to decrease parasitic capacitance between a data line layer and the pixel electrode. Accordingly, the pixel electrode 117 may be overlapped with the data line to realize a high aperture ratio. However, if the organic insulating layer of BCB is used for the passivation layer, the passivation layer 116 becomes thick. Thus, it is impossible to use the passivation layer of the organic insulating layer for a small-sized LCD device such as a mobile phone.
In this regard, in a small-sized LCD device, the passivation layer is formed of the inorganic insulating material such as silicon nitride SiNx. However, when using a passivation layer made of an inorganic insulating material, the parasitic capacitance increases greatly as compared with the passivation layer made of the organic insulating material, so that it is impossible to overlap the pixel electrode with the data line. If the parasitic capacitance increases between the data line and the pixel electrode, the parasitic capacitance causes a D.C. voltage offset relative to an A.C. voltage applied to the liquid crystal layer, ΔVp, thereby generating flicker, image sticking, and non-uniformity of luminance in the images.
Furthermore, the color filter array substrate 121 is formed opposite to the thin film transistor array substrate 111. The color filter array substrate 121 includes a color filter layer 123 of R(red)/G(green)/B(blue) pigments arranged in order, a black matrix layer 122 for dividing R/G/B cells and preventing light leakage, and a common electrode 124 for applying the voltage to the liquid crystal layer 131.
In the color filter layer 123, generally, pixels having the R/G/B pigments are arranged in order, wherein each sub-pixel has one pigment, and the sub-pixels are separately driven, thereby displaying a color in one pixel by compound of the sub-pixels.
Generally, the black matrix layer 122 is formed to correspond with the edge of the sub-pixel and the thin film transistor of the thin film transistor array substrate, thereby preventing the light leakage on the portions having an unstable electric field.
As described above, in case of using the passivation layer of the inorganic insulating material, it is impossible to overlap the pixel electrode with the data line. Accordingly, the black matrix layer 122 is overlapped with the pixel electrode 117 to prevent the light leakage between the pixel electrode and the data line. At this time, because the pixel electrode 117 is formed on the thin film transistor array substrate, and the black matrix layer 122 is formed on the color filter array substrate, it is necessary to provide a bonding margin in the black matrix layer 122 to prevent the light leakage. The bonding margin may be varied on bonding apparatus. However, as illustrated in FIG. 2, preferably, the black matrix layer 122 is overlapped with the pixel electrode 117 by a minimum of 5 μm to 6.7 μm.
However, the related art LCD device has the following disadvantages.
First, the passivation layer is formed of the inorganic insulating material, the pixel electrode is not overlapped with the adjacent data line. Accordingly, there is a requirement for providing the sufficient bonding margin to overlap the black matrix layer with the pixel electrode, thereby preventing the light leakage between the pixel electrode and the data line. Thus, the size of the black matrix layer increases due to the bonding margin, so that the aperture ratio of the LCD device lowers.
If the passivation layer is formed of the inorganic insulating layer, the pixel electrode is formed not to overlap the adjacent data line. In this state, the dielectric constant of the passivation layer is higher than the dielectric constant of the organic insulating layer, so that parasitic capacitance is generated between the pixel electrode and the data line. Because of the parasitic capacitance, a source delay is generated which decreases a data voltage level. Thus, the luminance is changed due to the source delay, so that vertical crosstalk generates, thereby deteriorating the picture quality.